This invention relates to methods and apparatus for investigating earth formations traversed by a borehole, and more particularly relates to methods and apparatus for achieving range control of formation measurements.
It is well known that many of the parameters which are desirably measured in a well logging operation vary over large dynamic ranges from borehole to borehole and within a particular borehole itself due to variations in formation characteristics sought to be investigated.
As but one example, in the case of electrical logging, it has been found that formation resistivities encountered may vary from 0.2 ohm-meters to over 10,000 ohm-meters. In the early instruments for measuring such resistivities, typically a constant reference voltage (or current) was induced into the formation and a measure voltage which thus varied as a function of changing formation resistivities was detected. One exemplary type of such instrument is disclosed, for example, in U.S. Pat. No. 2,712,627 to H. G. Doll.
However, it was found that as the formation conductivity varied from 5 MHO/m to 0.5 mMHO/m this measure voltage would typically vary over a 10,000:1 ratio, exceeding the dynamic range capability of even the best instrumentation amplifiers and the like, particulary in the adverse conditions encountered in the borehole. Clearly, some means was desirable to reduce the necessity of providing downhole circuitry associated with measurement of these parameters which would maintain accuracy over such large signal ranges.
One such attempt to reduce the aforementioned dynamic range problem is disclosed in U.S. Pat. No. 2,776,402 to F. P. Kokesh. The approach essentially was to employ a surveying current which diminished systematically as formation resistivity increased, resulting in measurements of resistivity which became non-linear as full scale value was approached. This approach is not unlike the attempt of logging operators in the past to manually adjust survey currents during logging operations, a practice which was fraught with difficulties which included variation in operator response time (resulting in lost data), lack of recording the magnitude of current changes rendering absolute resisitivity measurements impossible and the like.
While this technique tended to reduce the dynamic range problem somewhat, a major disadvantage was that it required anticipation of the formation resistivity range to be encountered, in that the value of a resistive means utilized to reduce the survey current was selected in accordance therewith.
Yet another approach was taught by L. Henry, et al in U.S. Pat. No. 3,539,910. In this technique, means were provided for adjusting the survey current so as to maintain the product of the current and the resulting measured voltage constant (i.e., a constant power system), resulting in a dynamic range reduction of a square root factor.
This approach however also suffered from major disadvantages, only one of which being, for example, that multiplying circuitry for providing the current-voltage product was typically extremely sensitive to temperature variations and other conditions of the deleterious environment of a borehole, rendering their application impracticable
Yet another problem with the previous attempts to achieve a range control in well logging devices particularly of the resistivity measuring type is that any such adjustments in gains were generally made based upon current parameter measurements and were thus relatively unsophisticated in a sense that a decision to change gain was based upon a relatively simple criteria which was invariant and thus insensitive to previously measured parameters. Provision has not been made therefore for adaptive gain ranging wherein the gain may be adjusted as a function of a history of plurality of prior measurements.
Although some prior devices such as those discussed above achieve such range control by adjusting current or voltage signals, they do not record the magnitudes of such gain adjustments. Such recorded adjustments may be used to obtain representation of actual formation resistivities rather than merely relative changes in resistivity.
Thus, the prior devices, by not making use of the adjustment magnitudes, thereby discard valuable information which may be used to more particularly determine the nature of the formation being investigated. The present invention overcomes this disadvantage by recording and utilizing the magnitudes of these adjustments.
With respect to inductive-type formation measurements, it is conventional to introduce into the formation, by means of an appropriate transmitter coil, an alternating field having a nominal frequency of, for example, 20 KHz, thereby inducing eddy currents into the formation. Fields from the induced eddy currents induce signals in a receiver coil which are thereafter routed to a phase sensitive detector so as to measure only those voltages corresponding to the induced currents and formation characteristic to be measured, thus rejecting, e.g., the relatively large voltage which is 90.degree. out of phase therewith corresponding to voltage induced in the receiver from transformer-type coupling from the transmitter coil to the receiver coil.
In the previously described formation measuring techniques in which current directly introduced therein by an electrode pad or "button" is measured, noise rejection frequently is not as problematical as in induced currenttype measurements. One reason for this is that the aforementioned receiver-transmitter mutual transformer coupling noise signal, which is 90.degree. out of phase from the desired eddy current signal and relatively large with respect thereto, is not present in such other systems. Thus, detection techniques other than phase detection, such as peak envelope detection, may be successfully employed.
One problem associated with such measurements of induced current is that typically the accuracy of phase detectors known in the art is sensitive to the magnitude of the dynamic amplitude range or swing of its input signal. Thus, prior art devices, by not seeking to automatically control measurement signal amplitudes seen by the phase detector were limited in the resolution of the desired measurement signals.
The disadvantages of the prior art are overcome by the present invention and the improved methods and apparatus are provided for achieving range control of measurements resulting from earth formation investigating apparatus.